1. Field of the invention
This invention relates to antenna receiving apparatus for receiving or transmitting radio signals and, more particularly for receiving or transmitting orthogonally polarized radio signals.
2. Description of the Prior Art
A crowding of the electro-magnetic frequency spectrum for the transmission of radio signals, particularly in satellite transmissions, has lead to the practice of frequency reuse, which allows for a double use of the frequency spectrum by simultaneously transmitting two signals using the same frequency bandwidth, but with their polarizations oriented at ninety degrees, or in an orthogonal relationship, to each other.
One mechanism for transmitting or receiving orthogonally polarized signals is to use an Ortho-Mode Transducer or OMT, which is used to couple two orthogonally polarized signals to a common junction. The OMT can be constructed with the common and the two orthogonal ports as waveguide ports, or with the common port as waveguide and the two orthogonal ports as coaxial ports, or with the common port as waveguide and one orthogonal port as waveguide and the other orthogonal port as coaxial. The common junction is usually constructed of waveguide and is an integral part of the feed in antenna systems.
Waveguide transmission lines come in an assortment of types including circular, square, elliptical, rectangular, and coaxial with the most commonly used types for OMTs being either circular or square. Since waveguides can support a multitude of transmission modes, the cross-sectional dimensions of the waveguide are normally selected such that only the lowest order or primary mode is allowed to propagate within the desired frequency band. In circular waveguide the TE11 mode is the primary mode with the TM01 mode as the first higher order mode. Each mode will have its own electric field (E-Field) and magnetic field (H-Field) arrangement. The other waveguide types, including square and coaxial, have similar types of modes with their own unique designations.
Waveguide modes are described as either TE for Transverse-Electric or TM for Transverse-Magnetic modes. The expression transverse is used to describe fields which are orthogonal to the direction of propagation. For example a TE mode in a waveguide would have its electric fields arranged such that they were orthogonal to the direction of propagation or the longitudinal axis of the waveguide. The numbers following TE or TM such as TM01 designate the order of the mode. A third type of mode is the TEM for Transverse Electro-Magnetic mode which is a non waveguide mode and can only propagate between two conductors which do not have contact with each other. The TEM mode has both its electric and magnetic fields transverse to the direction of propagation. The TEM mode is the mode of propagation for coaxial cable, micro-strip board, and stripline board. Since only one TEM mode exists, no numbers are used to describe the order of the mode.
Coaxial waveguides differ from the other waveguide shapes in that they have an outer and a center conductor which can support the coaxial mode in addition to the waveguide modes. The coaxial mode, which is often referred to as the TEM mode or Transverse Electro-Magnetic mode, is not a waveguide mode and can propagate in coaxial waveguide independent of the waveguide dimensions. Coaxial waveguides come in a variety of cross-sectional shapes including circular, square, elliptical, and rectangular. Since the coaxial or TEM mode is dependent on the presence of a center conductor, it does not exist in the other waveguide configurations discussed in the previous paragraphs.
Two types of probes are commonly used to couple signals from a waveguide transmission line to coaxial lines. The first is an electric field (E-Field) probe, which passes through the waveguide wall and extends into, but does not contact, the waveguide, such that it is aligned with or parallel to the electric fields of the primary mode in the waveguide. The second probe type is the magnetic field (H-Field) probe, which passes through the waveguide wall and extends into the waveguide where it is grounded to the waveguide wall forming a magnetic coupling loop. It is aligned such that the magnetic fields pass through the grounded loop. The electric field probes are normally preferred because they are simpler to construct and usually perform better than the magnetic field probes. However, the principles of this invention apply equally to both E-Field and H-Field probes.
A common method for constructing dual probe OMTs is to have two E-Field probes enter through the sidewalls of the waveguide. The two probes are located in separate planes normal to the waveguide axis and will be situated in a ninety degree or orthogonal relationship to each other. Typically, a first probe will be located one-quarter of a wavelength from the back wall, while the second probe will be three-quarters of a wavelength from the back wall.
When receiving signals, this arrangement can present difficulties, since it is often desirable to have both signals routed to the same circuit board where the low-noise amplifiers ("LNA's") are located, or to a switch which alternately switches the first and the second probe to a single LNA. The line length between each probe and its LNA must be minimized to preserve the lowest noise figure possible for the system. Furthermore, the probes will tend to be limited by narrow band responses, particularly the one located the greater distance from the back wall.
When two or more probes are located in the same axial plane, they usually interact with each other, causing the signal from one probe to couple to the other probe(s). The mechanism for this cross coupling is usually the excitation of higher order modes, which can be excited as non-propagating or evanescent modes. An E-Field probe extending through the side wall of a circular waveguide will tend to excite the TM01 mode in addition to the primary or TE11 mode. Since the TM01 mode has electric fields which are aligned with both probes, cross coupling can result. Since the TM01 mode is a non-propagating mode in most cases, the second probe can be isolated by positioning it some distance away from the first probe.
Donald C. Cloutier (U.S. Pat. No. 4,595,890) discloses two E-Field probes extending through the back wall of a circular waveguide. This probe arrangement normally would suffer from cross coupling. However, in Cloutier's case each probe is in turn deactivated by diode switches located in the waveguide and connected directly to the probes.
Mon N. Wong et al (U.S. Pat. No. 4,965,868), by positioning four co-planar E-Field probes in a circular waveguide, was able to minimize the problem of cross coupling. Since each of the four probes will excite the TM01 mode along with the TE11 mode, there will be substantial cross coupling between all four probes. However, with the use of four probes, Wong is able to eliminate the cross-coupled component in a summing network connected to each diagonal pair of probes.
Hiroshi Kume et al (Japan Pat. #56-8301) recognized the benefits of routing the E-Field probes through the back wall of a circular waveguide. However, Kume, like Wong, resorted to the use of four probes and summing networks to minimize the effects of cross coupling between the probes.